The present invention relates to a polyazole-containing composition in the form of a solution and/or dispersion, to a process for preparation thereof and to the use thereof, especially for production of membrane electrode assemblies for fuel cells.
Polymer electrolyte membranes (PEMs) are already known and are especially used in fuel cells. Frequently, sulfonic acid-modified polymers, especially perfluorinated polymers, are employed. A prominent example thereof is Nafion™ from DuPont de Nemours, Willmington USA. For proton conduction, a relatively high water content in the membrane is required, which is typically 4-20 molecules of water per sulfonic acid group. The necessary water content, but also the stability of the polymer in conjunction with acidic water and the hydrogen and oxygen reaction gases, limits the operating temperature of the PEM fuel cell stack typically to 80-100° C. Under pressure, the operating temperature can be increased to >120° C. Otherwise, higher operating temperatures cannot be achieved without a loss in performance of the fuel cell.
For system reasons, however, higher operating temperatures than 100° C. in the fuel cell are desirable. The activity of the noble metal-based catalysts present in the membrane electrode unit (MEU) is significantly better at high operating temperatures. More particularly, in the case of use of what are called reformates from hydrocarbons, distinct amounts of carbon monoxide are present in the reformer gas, which typically have to be removed by complex gas treatment or gas purification. At high operating temperatures, the tolerance of the catalysts to the CO impurities rises up to several % by volume of CO.
In addition, heat evolves in the operation of fuel cells. Cooling of these systems to below 80° C. can, however, be very costly and inconvenient. According to the power output, the cooling apparatuses can be made much simpler. This means that, in fuel cell systems which are operated at temperatures above 100° C., the waste heat can be utilized much better, and hence the fuel cell system efficiency can be enhanced by power-heat coupling.
In order to attain these temperatures, membranes with novel conductivity mechanisms are generally used. One approach for this purpose is the use of membranes which exhibit electrical conductivity without the use of water. A first development in this direction is detailed, for example, in WO 96/13872. For instance, WO 96/13872 discloses the use of acid-doped polybenzimidazole membranes which are produced by a casting process.
A new generation of acid-containing polyazole membranes which likewise exhibit electrical conductivity without the use of water is described in WO 02/088219. This application discloses a proton-conducting polymer membrane based on polyazoles, which is obtainable by a process comprising the following steps:    A) mixing one or more aromatic tetramino compounds with one or more aromatic carboxylic acids or esters thereof which comprise at least two acid groups per carboxylic acid monomer, or mixing one or more aromatic and/or heteroaromatic diaminocarboxylic acids, in polyphosphoric acid to form a solution and/or dispersion    B) applying a layer using the mixture according to step A) on a carrier, optionally on an electrode,    C) heating the flat structure/sheet obtainable according to step B) under inert gas to temperatures of up to 350° C., preferably up to 280° C., to form the polyazole polymer,    D) treating the membrane formed in step C) until it is self-supporting, preferably by partial hydrolysis.
The polyphosphoric acid used in step A) typically has a content, calculated as P2O5 (by acidimetric means), of at least 83%.
To adjust the viscosity, the solution can optionally be admixed with phosphoric acid (conc. phosphoric acid, 85%).
The examples describe numerous syntheses in a polyphosphoric acid having a content, calculated as P2O5 (by acidimetric means), of 83.4%. Some of the batches are diluted with conc. phosphoric acid.
The content of the resulting solutions, calculated as P2O5 (by acidimetric means), is either at most 70.487752% (=theoretical H3PO4 concentration: 97.3%; example 5) or at least 75.465388% (=theoretical H3PO4 concentration: 104.2%, example 3).
The intrinsic viscosity of the polymers at 30° C. is 2.9 dl/g or less.
The acid-containing polyazole membranes disclosed in WO 02/088219 exhibit a favorable profile of properties per se and are especially suitable for use in membrane electrode assemblies for fuel cells.
However, the solutions and/or dispersions obtainable in step A) have a high viscosity, especially for relatively high solids contents. The same applies to the polymer solutions or dispersion obtainable by polymerizing the monomers, the viscosity of the solution or dispersions rising further with rising degree of polymerization.
The processing of the solutions or dispersions therefore frequently requires relatively high temperatures. However, the problem is confronted here that the viscosity of the solutions and/or dispersions continues to rise constantly with time above 170° C.
In addition, the hydrolysis in step D) proceeds relatively slowly. Furthermore, the production of thin and/or defect-free membranes or self-supporting films/flat structures is possible only with difficulty.
Finally, membranes with better mechanical properties, especially a higher tensile strength and an improved mechanical stability, are desirable.